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Investments (Investments Held in a Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fair value
Fair Value	$ 3,221,782	$ 3,333,683
Less than 12 months, Fair Value	573,901	1,300,381
12 months or longer, Fair Value	2,647,881	2,033,302
Unrealized Loss
Unrealized Loss	387,610	381,339
Less than 12 months, Unrealized Loss	40,866	84,907
12 months or longer, Unrealized Loss	346,744	296,432
U.S. Treasury obligations
Fair value
Fair Value	237,828	220,991
Less than 12 months, Fair Value	53,724	53,199
12 months or longer, Fair Value	184,104	167,792
Unrealized Loss
Unrealized Loss	23,239	22,399
Less than 12 months, Unrealized Loss	2,863	2,393
12 months or longer, Unrealized Loss	20,376	20,006
U.S. Government-sponsored enterprise obligations
Fair value
Fair Value	18,871	19,934
Less than 12 months, Fair Value	4,311	8,082
12 months or longer, Fair Value	14,560	11,852
Unrealized Loss
Unrealized Loss	1,483	1,628
Less than 12 months, Unrealized Loss	352	663
12 months or longer, Unrealized Loss	1,131	965
State and municipal bonds
Fair value
Fair Value	417,930	421,769
Less than 12 months, Fair Value	84,695	177,393
12 months or longer, Fair Value	333,235	244,376
Unrealized Loss
Unrealized Loss	44,058	44,311
Less than 12 months, Unrealized Loss	4,940	12,352
12 months or longer, Unrealized Loss	39,118	31,959
Corporate debt
Fair value
Fair Value	1,600,097	1,708,529
Less than 12 months, Fair Value	186,646	687,947
12 months or longer, Fair Value	1,413,451	1,020,582
Unrealized Loss
Unrealized Loss	200,768	199,862
Less than 12 months, Unrealized Loss	12,441	42,977
12 months or longer, Unrealized Loss	188,327	156,885
Residential mortgage-backed securities
Fair value
Fair Value	370,223	363,945
Less than 12 months, Fair Value	140,460	155,212
12 months or longer, Fair Value	229,763	208,733
Unrealized Loss
Unrealized Loss	73,236	61,656
Less than 12 months, Unrealized Loss	17,086	15,275
12 months or longer, Unrealized Loss	56,150	46,381
Agency commercial mortgage-backed securities
Fair value
Fair Value	8,652	9,704
Less than 12 months, Fair Value	821	3,086
12 months or longer, Fair Value	7,831	6,618
Unrealized Loss
Unrealized Loss	1,116	872
Less than 12 months, Unrealized Loss	29	110
12 months or longer, Unrealized Loss	1,087	762
Other commercial mortgage-backed securities
Fair value
Fair Value	183,434	192,359
Less than 12 months, Fair Value	20,378	53,270
12 months or longer, Fair Value	163,056	139,089
Unrealized Loss
Unrealized Loss	24,169	22,994
Less than 12 months, Unrealized Loss	1,836	4,087
12 months or longer, Unrealized Loss	22,333	18,907
Other asset-backed securities
Fair value
Fair Value	384,747	396,452
Less than 12 months, Fair Value	82,866	162,192
12 months or longer, Fair Value	301,881	234,260
Unrealized Loss
Unrealized Loss	19,541	27,617
Less than 12 months, Unrealized Loss	1,319	7,050
12 months or longer, Unrealized Loss	$ 18,222	$ 20,567

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